This invention pertains to plasma spraying techniques and particularly to systems and methods utilizing transfer arcs in a supersonic plasma stream.
Plasma spray processes are commercially used for coating precision parts with metals and ceramics that are resistant to high temperatures, wear, corrosion or other conditions. Plasma sprayers provide a high energy level stream of ionized gas that can heat a workpiece to a high temperature and also deposit a powder of a selected coating material onto the workpiece. The powder is injected into the plasma stream and is heated to a molten or plastic state and bonded upon impact to a preferably heated workpiece. In the present state of the art, coatings can be provided having densities of 70 to 90% of theoretical, with the bond between the coating and substrate being of a mechanical rather than a chemical or metallurgical nature. It is desirable to increase the average coating density and the strength of the bond, and also to improve the yield using the process. Yields are sometimes uncertain, and generally less than satisfactory, because the dynamics of the process are dependent upon a number of variables involving high energy levels that cannot be precisely controlled, such as the stream velocity, plasma temperature and pressure conditions. The density of the coating and the strength of the bond are dependent not only on these variables but also on the cleanliness and condition of the workpiece.
Transferred arc type plasma guns have been used for powder overlay coatings and more recently for powder spray coatings. In these types of devices, a primary cathode-anode arc within the gun creates the plasma by ionizing a gas stream, and a potential difference between the gun itself and the workpiece serves to establish the workpiece as an anode to which the transfer arc from the gun attaches. Because the arc normally attaches within a very small area on the workpiece, tending to erode the surface and restricting the deposition rate, some modern plasma spray systems operate in a fashion to create an arc diffusing shock pattern. A supersonic plasma stream is created, but the stream static pressure is held relatively low, approximately 1 atmosphere, by a pumping system coupled into the enclosure for the device. Using a plasma stream velocity in the range of Mach 2 to 3, the shock pattern on the workpiece distributes the arc and spreads the powder during deposition. The high gas and powder velocities, and the consequent increase in kinetic and mechanical impact energies of the coating material, produce coatings with improved densities (in the range of 96 to 99% of theoretical) and improved bond strengths. The expansion of the stream due to the dynamic pressure ratios also substantially increases the area over which coating is deposited on the workpiece. However, control over the process is still far less than ideal, again primarily because of the dynamnic nature of the process. In heating the workpiece with the plasma stream, for example, nonuniform buildup can occur and some oxidation can take place, reducing the integrity of the bond and effecting the rate of deposition of material. The presence of oxidation or other impurities on the part severely affects quality, and precleaning techniques do not resolve the problem. Also it is desirable to use a commerical gas, rather than a much more costly purified gas, for the plasma system. The stringent requirements and demands that are placed on parts, such as turbine blades, that are typically coated by this process in turn means that the parts must be rejected in quality control.